The present invention relates to a process for producing a heat-resistant porous polyimide having a finely cellular structure and to a porous polyimide obtained by the process. This porous polymer is extremely useful as, for example, circuit substrates for electronic appliances, etc.
Polyimide resins have conventionally been used widely as parts or members required to have reliability, such as circuit substrates for electronic/electrical appliances and other electronic parts, because of their features such as high insulating properties, dimensional stability, moldability, and lightweight properties. Especially in recent years, there is a desire for higher-speed information transmission with the trend toward performance and function advancement in electrical/electronic appliances, and members for use in these appliances also are required to be compatible with the desired higher-speed information transmission. The polyimides for use in such applications are also required to have a lower dielectric constant and a smaller dielectric dissipation factor as electrical properties necessary for the higher-speed transmission.
In general, the dielectric constant of a plastic material is determined by the molecular framework thereof. This means that a technique which may be effective in reducing dielectric constant is to modify a molecular framework. However, in view of the fact that the dielectric constants of polyethylene and polytetrafluoroethylene, which are regarded as low-dielectric-constant polymers, are about 2.3 and about 2.1, respectively, there are limitations in the technique of controlling dielectric constant based on molecular structure modifications. In addition, the above technique poses problems, for example, that a molecular structure modification results in changes in properties such as film strength and coefficient of linear expansion.
As other attempts to obtain a lower dielectric constant, various techniques have been proposed in which a plastic material is made porous so as to utilize air, which has a dielectric constant of 1, and to control the dielectric constant of the plastic based on the porosity.
Conventional techniques for obtaining general porous polymers include dry processes and wet processes. Known dry processes include a physical foaming method and a chemical foaming method. In the physical foaming method, a low-boiling solvent such as, e.g., a chlorofluorocarbon or hydrocarbon is dispersed as a blowing agent into a polymer and this polymer is then heated to volatilize the blowing agent. Thus, cells are formed to obtain a porous polymer.
In the chemical foaming method, a blowing agent is added to a polymer and then pyrolized. Thus, a gas is generated to form cells and thereby obtain a foam.
The physical foaming technique has problems concerning the harmfulness of the substances to be used as blowing agents and various influences thereof on the environment, e.g., ozonosphere depletion. In addition, although the physical technique is generally suitable for use in producing foams having a cell diameter of tens of micrometers or larger, it is difficult with this technique to obtain a foam having fine cells with uniformity in diameter. On the other hand, the chemical foaming technique has a drawback that the blowing agents, after gas generation and foaming, leave residues in the resultant foams. Such corrosive gases and impurities pose a problem concerning pollution especially in applications such as electronic parts, where pollution reduction is highly required.
Recently, a technique for obtaining a porous object having a small cell diameter and a high cell density has been proposed. This technique comprises dissolving an inert gas such as nitrogen or carbon dioxide in a polymer at a high pressure, subsequently releasing the polymer from the pressure, and heating the polymer to around the glass transition temperature or softening point thereof to thereby form cells. This foaming technique, in which cells are formed by generating nuclei from the gas in a thermodynamically unstable state and then expanding and growing the nuclei, has an advantage that a foam having microporosity which has been unobtainable so far can be obtained.
In JP-A-6-322168 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) is proposed a process for producing a heat-resistant foam by applying the technique described above to a polyetherimide as a thermoplastic polymer. However, this process has the following drawback. When a polymer is impregnated with a high-pressure gas in a pressure vessel, the pressure vessel is heated to or around the Vicat softening point of the polymer. Because of this heating, the polymer is in a molten state during pressure reduction and, hence, the high-pressure gas readily expands. As a result, the foam obtained has a cell size as large as from 10 to 300 xcexcm. Consequently, this foam, when intended to be used as a circuit substrate, needs to have a large thickness and imposes limits on the formation of finer patterns.
On the other hand, JP-A-10-45936 proposes a technique of forming a foamed molding having closed cells with a cell size of from 0.1 to 20 xcexcm by likewise applying that technique to a styrene resin having a syndiotactic structure, and further proposes use of the foamed molding as an electric circuit member. However, since styrene resins having a syndiotactic structure generally have a glass transition point around 100xc2x0 C., this foamed molding deforms or bends when used at temperatures of 100xc2x0 C. or higher. Consequently, this foamed molding is usable only in a limited range of applications. Furthermore, in the techniques for imparting porosity by physical or chemical foaming, there is a possibility that the polymer being foamed might suffer delamination. None of the references cited above discloses use of a polyimide.
One object of the invention is to provide a process for producing a porous polyimide having a finely cellular structure and having a low dielectric constant and heat resistance.
Another object of the invention is to provide the porous polyimide.
The present inventors made investigations in order to overcome the problems described above. As a result, they have found that when a dispersible compound for forming a discontinuous phase is added to a polyimide precursor serving as a continuous phase to form a specific micro-domain structure in the polymer and is subsequently removed therefrom by, for example, an extraction operation with supercritical carbon dioxide or the like, then a porous object having extremely fine cells and a low dielectric constant can be obtained. The invention is based on this finding.
The invention provides a process for producing a porous polyimide which comprises preparing a polymer composition having a micro-domain structure made up of a continuous phase comprising a polyimide precursor A and, dispersed therein, a discontinuous phase which comprises a dispersible compound B and has an average diameter smaller than 10 xcexcm, removing the dispersible compound B from the polymer composition, and then converting the polyimide precursor A into a polyimide, an interaction parameter "khgr"AB between the polyimide precursor A and the dispersible compound B being larger than 3.
In the process described above, the removal of the dispersible compound B may be conducted by an extraction operation. In this case, it is preferred to use supercritical carbon dioxide as an extraction solvent.
The invention further provides a porous polyimide which is obtained by the above process and has an average cell diameter smaller than 5 xcexcm and a dielectric constant of 3 or lower.